Control systems having data networks are used inter alia in automation engineering for controlling automation processes. In this case, the data network connects a control device of the control plane to signal units of the sensor/actuator plane or field plane. The signal units normally have inputs for connecting sensors via which input signals can be read in that characterize the state of the automation process. Such state signals can represent a position or speed of machine parts, a position of a switch, a pressure or a temperature. Alternatively or additionally, the signal units have outputs for connecting actuators via which output signals can be output to the automation process in order to alter the state of the automation process. Such actuators may be motors, valves or heating elements, for example.
In the signal units, the input signals are converted into input data and the input data are subsequently transmitted to the control device via the data network. The control device evaluates the input data in order to determine the state of the automation process to be controlled, and takes the input data as a basis for generating output data in order to influence the automation process in a desired manner. The output data are transmitted to the signal units via the data network, and the signal units take the output data as a basis for outputting the output signals to the actuators.
The control of the automation process is normally effected in successive control cycles. During a control cycle, output data are computed in the control device and provided for output to the automation process. Subsequently, the output data are transmitted from the control device to the signal units via the data network and input data are transmitted from the signal units to the control device via the data network. In a subsequent control cycle, the input data are used for computing new output data in the control device, while the signal units take the output data as a basis for outputting the output signals.
The computation of the output data in the control device, and the reading-in of the input signals and the output of the output signals in the signal units, are normally effected during a computation cycle of the control cycle, while the transmission of the output data and the transmission of the input data via the data network are effected during a communication cycle of the control cycle. The duration of the control cycle determines how quickly it is possible to react to state changes for the automation process. The duration or cycle time of the control cycle is inter alia determined by the time that is needed for executing the communication cycle, that is to say for the data transmission of the output data from the control device to the signal units and for the data transmission of the input data from the signal units to the control device. For the data transmission, the time that elapses between data in the transmitter being transmitted and the data being received in the predetermined receiver is also referred to as the latency of the data transmission.
The latency of the data transmission substantially determines the cycle time of the automation process control and hence the speed at which it is possible to react to status changes for the automation process. The shorter the cycle time or the lower the latency, the more quickly the outputs of the control system can be updated and the more accurately the automation process can be controlled.
The latency of the data transmission via the data network is dependent inter alia on the number of network components through which the data sent via the data network need to pass on their way between transmitter and receiver. Additionally, the network components themselves can contribute to increasing the latency, for example if they first of all process directed data, buffer-store them or convert them between different data or protocol formats.
A low latency is in particular needed for controlling automation processes in which control loops are completed via the data network. This is in particular the case for drives in which a mobile element of the drive, for example a rotor of an electric motor, is moved by means of an actuator connected to a signal unit of the control system and the position of the mobile element is regulated by the control device on the basis of position data that are obtained from another signal unit of the control system. In such systems, the latency of the data transmission on the data network can substantially determine the possible bandwidth of the control loop.